Chemical Oxygen Demand (COD) is the single most important indicator of organic pollution in textile printing and dyeing wastewater, and its accurate determination is essential for regulatory compliance, treatment process control, and water reuse evaluation. This article provides a comprehensive technical overview of COD testing for dyeing effluent, covering applicable standards such as GB 4287-2012, HJ 828-2017, HJ/T 399-2007, and ISO 15705, sampling and preservation requirements, dichromate reflux titration, rapid digestion spectrophotometry, interference control, and quality assurance practices.
Technical Overview
Chemical Oxygen Demand (COD) is defined as the mass of oxygen, expressed in mg/L, equivalent to the amount of reducible matter (primarily organic compounds, plus some inorganic species such as sulfides and ferrous iron) that is chemically oxidized by potassium dichromate in a hot, strongly acidic medium. Because textile printing and dyeing effluent contains large quantities of dyes, hydrolyzed dye residues, sizing agents, surfactants, auxiliaries, and organic acids, COD serves as the primary regulatory and engineering indicator of its pollution strength.
Printing and dyeing wastewater is generated from multiple process units, each contributing a characteristic organic load:
- Desizing effluent, rich in starch, polyvinyl alcohol (PVA), and polyacrylate sizes, frequently exceeding 5,000 mg/L COD and often the strongest single stream in a mill.
- Scouring and mercerizing effluent, carrying waxes, pectins, alkali, and detergents.
- Bleaching wash water containing residual peroxide and stabilizers.
- Dyeing and printing effluent containing reactive, disperse, azo, and vat dyes, urea, thickeners, binders, and high concentrations of sodium chloride or sodium sulfate from reactive dyeing.
- Washing and finishing streams with detergents, softeners, and finishing agents.
Combined raw effluent typically exhibits COD values of 400–2,500 mg/L, strong color, fluctuating pH, high salinity, and a relatively low BOD5/COD ratio of approximately 0.2–0.35, which indicates moderate biodegradability and usually requires a combination of biological and advanced treatment. Under the Chinese national discharge standard GB 4287-2012 (Discharge Standard of Water Pollutants for Dyeing and Finishing of Textile Industry), the CODCr limit is 80 mg/L for direct discharge to receiving waters and 200 mg/L for indirect discharge to municipal sewers or centralized wastewater treatment plants, with stricter special emission limits applicable in designated environmentally sensitive regions.
Testing Samples
Untreated Raw Effluent
Composite or grab samples collected from workshop outfalls, the general drainage channel, or the inlet of the equalization basin. These samples characterize the total pollution load and are used for treatment plant design, pollution load accounting, and discharge permit management.
In-Process Streams at Each Treatment Stage
Samples taken after unit operations such as equalization, screening, hydrolytic acidification (anaerobic pretreatment), aerobic biological treatment (activated sludge, oxidation ditch, or membrane bioreactor), secondary sedimentation, coagulation and flocculation, advanced oxidation (Fenton, ozone, or electrochemical oxidation), and adsorption polishing. Stage-by-stage COD profiles reveal the removal efficiency of each unit and guide optimization of chemical dosages and aeration intensity.
Final Discharged Effluent
Samples collected at the standardized discharge outfall or monitoring well, representing the legally regulated discharge point. These results are compared directly against the applicable limit values in GB 4287-2012 and the facility's discharge permit.
Reclaimed and Reuse Water
Effluent from advanced treatment trains, including ultrafiltration and reverse osmosis permeate intended for reuse in dyeing, rinsing, or boiler feed. Reuse applications typically demand very low COD (commonly below 50 mg/L), minimal color, and low conductivity, so accurate low-level COD determination is critical.
Testing Items
The core testing item is CODCr, the dichromate-reflux chemical oxygen demand, which is the statutory indicator under GB 4287-2012. In practice, COD determination is accompanied by a suite of related parameters that together characterize dyeing wastewater quality:
- CODCr (dichromate method), including on-line automatic monitoring data
- BOD5, used to derive the BOD5/COD ratio and assess biodegradability
- pH, suspended solids (SS), and color (dilution factor)
- Ammonia nitrogen, total nitrogen, and total phosphorus
- Sulfide, aniline compounds, and hexavalent chromium, which are listed in the discharge standard
- Total organic carbon (TOC), sometimes used as a supplementary or correlative parameter
Testing Methods
Several internationally and nationally recognized methods exist for COD determination. The most widely applied in dyeing wastewater testing are the dichromate reflux titration method, the rapid digestion–spectrophotometric (sealed-tube) method, and the chlorine-correction method for high-salinity samples.
Sample Collection and Preservation
Representative samples are collected in clean glass containers. For compliance monitoring, 24-hour time-proportional or flow-proportional composite samples are preferred. Immediately after collection, samples are acidified with concentrated sulfuric acid to pH ≤ 2 to inhibit microbial degradation and refrigerated at approximately 4 °C. Analysis should be performed as soon as possible, generally within 48 hours, in accordance with the applicable water sampling and preservation specifications. Before subsampling, the container must be shaken thoroughly to resuspend settleable solids and fibers so that the aliquot is homogeneous.
Dichromate Reflux Titration Method
The reference method, specified in HJ 828-2017 (which superseded GB 11914-89) and consistent in principle with ISO 6060:1989 and ASTM D1252, is as follows. A measured aliquot of sample is introduced into a reflux flask together with mercuric sulfate (to complex chloride), a known excess of standard potassium dichromate solution, and a silver sulfate–sulfuric acid catalyst solution. The mixture is boiled under open reflux for two hours, during which organic matter is oxidized and a stoichiometric amount of hexavalent chromium is reduced to trivalent chromium. After cooling and dilution, the remaining dichromate is titrated with standardized ferrous ammonium sulfate using ferroin (1,10-phenanthroline–iron(II) complex) as the indicator, and a reagent blank is titrated in parallel. COD is calculated as the difference in titrant volumes multiplied by the titrant concentration, the oxygen equivalent factor of 8 mg per millimole, and an appropriate volume correction, divided by the sample volume.
Using a 0.250 mol/L potassium dichromate solution, the method is suitable for samples between approximately 10 mg/L (with the low-concentration 0.025 mol/L oxidant) and 700 mg/L without dilution; higher-strength streams, such as desizing effluent, are diluted with organic-free water before digestion. The complete procedure, including reflux, cooling, and titration, typically requires about four hours, which is why it serves as the arbitration and verification method.
Rapid Digestion–Spectrophotometric (Sealed-Tube) Method
Specified in HJ/T 399-2007 and aligned with ISO 15705:2002 (small-scale sealed-tube ST-COD), this method uses commercially prepared or laboratory-dosed digestion vials containing pre-measured potassium dichromate, silver sulfate catalyst in sulfuric acid, and mercuric sulfate. The sample is added to the vial and digested in a thermostatic heating block, commonly at approximately 165 °C for 20 minutes (or approximately 150 °C for 2 hours depending on the protocol). After cooling, absorbance is measured spectrophotometrically: in the high-range configuration, the decrease of orange hexavalent chromium is monitored at about 600 nm; in the low-range configuration, the increase of green trivalent chromium is monitored at about 440 nm. COD is read from a calibration curve established with potassium hydrogen phthalate (KHP) standards, with a correlation coefficient typically required to be at least 0.999.
The method offers substantially lower reagent consumption, shorter turnaround (roughly one hour), reduced acid hazard, and high throughput, making it the preferred routine technique for process control and large batch screening. Results should be periodically cross-checked against the reflux titration method, particularly for compliance reporting.
Chlorine Correction Method for High-Chloride Effluent
Reactive dyeing generates effluent with chloride concentrations that may reach several thousand to over ten thousand mg/L. Chloride is oxidized by dichromate to free chlorine, producing a positive bias that conventional mercuric sulfate masking cannot fully suppress above roughly 1,000 mg/L. For such samples, HJ/T 70-2001 is applied: the sample is digested with dichromate in a specialized apparatus in which the free chlorine generated from chloride oxidation is collected and quantified iodometrically, and the corresponding oxygen equivalent is subtracted from the apparent dichromate demand to obtain the corrected COD. Where feasible, dilution with chloride-free water to bring chloride within the tolerance of the standard methods is an accepted alternative, provided the diluted sample still exceeds the method detection limit.
On-Line Automatic Monitoring
Key discharge outfalls at larger dyeing and finishing facilities are commonly equipped with automatic on-line COD analyzers based on dichromate digestion with optical or titrimetric detection, conforming to the relevant technical specifications for COD water quality automatic analyzers. These systems provide continuous data for environmental supervision and early warning of treatment upsets, with periodic calibration and validation against laboratory reference measurements.
Interferences and Their Control
- Chloride: the most significant interference in high-salt dyeing effluent; controlled by mercuric sulfate complexation at a mass ratio of at least 10:1 to chloride, by sample dilution, or by the chlorine-correction procedure of HJ/T 70.
- Nitrite: consumes dichromate; may be eliminated by adding sulfamic acid before digestion.
- Reduced inorganic species (ferrous iron, sulfide): contribute to apparent COD; their contribution should be recognized or the species removed before analysis where the organic fraction alone is of interest.
- Refractory organics: pyridine and some aromatic nitrogen heterocycles and volatile aromatics are only partially oxidized; silver sulfate catalyst is essential for oxidizing straight-chain aliphatic compounds, and sealed-tube digestion helps retain volatile organics.
- Color and turbidity: raw dye solutions are intensely colored, but after dichromate digestion the matrix color is largely destroyed; spectrophotometric readings must be taken at the wavelengths specified by the method, and samples must be homogeneous.
Quality Assurance and Quality Control
Accredited third-party laboratories operating under ISO/IEC 17025 quality management systems apply the following controls to every analytical batch: a reagent blank carried through the full procedure; duplicate determinations with relative deviation within the method-specified limits; calibration verification using potassium hydrogen phthalate reference standards (theoretical oxygen demand of 1.176 mg O2 per mg KHP); matrix spike recoveries typically within 90–110 percent; daily standardization of the ferrous ammonium sulfate titrant; and verified calibration of balances, thermometers, digestion blocks, and spectrophotometers. Method detection limits and reporting limits are established and documented, and results are reported with dilution factors clearly stated.
Applications
COD determination for printing and dyeing wastewater supports a wide range of regulatory, operational, and technical objectives:
- Statutory compliance monitoring against GB 4287-2012 and facility discharge permits, including government-supervised outfall sampling.
- Design and day-to-day operation of wastewater treatment plants, including control of aeration, sludge retention time, coagulant dosing, and Fenton or ozone reagent consumption.
- Evaluation and comparison of treatment technologies such as hydrolytic acidification, membrane bioreactors, electrochemical oxidation, and advanced oxidation processes in pilot and full-scale studies.
- Water reuse programs, verifying that reclaimed effluent meets the low COD and color requirements for reuse in dyeing and rinsing operations.
- Cleaner production audits, pollutant load accounting, and industrial park effluent fee assessment based on pollution load.
- Investigation of treatment upsets, accidental discharges, and receiving-water pollution incidents.
- Academic and industrial research on the treatability and degradation kinetics of dye-containing effluents.
FAQ
Why is CODCr used as the statutory indicator instead of the permanganate index?
Potassium dichromate under acidic heating with a silver catalyst oxidizes the great majority of organic compounds, including many dyes and polymers that are common in textile effluent, whereas the permanganate index oxidizes only a fraction of the organic load. CODCr therefore gives a truer measure of total chemically oxidizable pollution and is the basis of the limit values in GB 4287-2012.
What COD levels are typical at each treatment stage?
Raw mixed effluent is commonly 400–2,500 mg/L, with desizing streams occasionally exceeding 5,000 mg/L. After biological treatment, COD generally falls to 100–200 mg/L, and advanced oxidation, coagulation, or adsorption polishing is required to reliably reach the 80 mg/L direct discharge limit or reuse-grade quality below 50 mg/L.
How is chloride interference from reactive dyeing handled?
For chloride up to roughly 1,000 mg/L, mercuric sulfate masking in the standard dichromate methods is effective. Above this level, samples are either diluted with organic-free and chloride-free water or analyzed by the chlorine-correction dichromate method of HJ/T 70-2001, in which the free chlorine produced during digestion is quantified and subtracted.
How should samples be collected and preserved before COD testing?
Collect representative grab or composite samples in glass containers, acidify immediately with sulfuric acid to pH ≤ 2, refrigerate at about 4 °C, keep away from light, and analyze as soon as possible, ideally within 48 hours. Homogenize the sample thoroughly before taking the analytical aliquot so that fibers and settleable solids are included proportionally.
Which method should be chosen: reflux titration or rapid spectrophotometry?
Rapid digestion spectrophotometry is well suited to routine, high-throughput process control because of its speed and low reagent consumption. Dichromate reflux titration is the reference and arbitration method and is preferred for compliance-critical determinations, unusual matrices, or whenever spectrophotometric results require independent confirmation.
What does a low BOD5/COD ratio indicate for treatment design?
A BOD5/COD ratio of about 0.2–0.35, which is typical of dyeing wastewater, indicates limited biodegradability due to refractory dyes and auxiliaries. This signals the need for hydrolytic acidification pretreatment to improve biodegradability, followed by advanced oxidation or adsorption as polishing steps to meet stringent discharge limits.
How long does a COD determination take?
The rapid sealed-tube spectrophotometric method can be completed in roughly one hour including cooling and measurement. The classical reflux titration method requires about four hours from the start of reflux through cooling and titration. On-line analyzers provide results at fixed intervals, typically every two to four hours, enabling continuous supervision of the discharge outfall.